drm/radeon/kms: add cvt mode if we only have lvds w/h and no edid (v4)

This fixes LVDS on some mac laptops without a panel edid.

v2 - Set proper mode type flags
v3 - Note that this is not neceesarily the exact panel mode,
but an approximation based on the cvt formula.  For these
systems we should ideally read the mode info out of the
registers or add a mode table, but this works and is much
simpler.
v4 - Update comments and debug message.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>